At about 20-23 minutes into the movie, look at the part where Shaun gets a Diet Coke from the fridge in the shop (the second time around, after everyone's infected). After he closes the fridge, he slips on what we're supposed to believe is blood, as signified by the hand stains on the fridge door. But if you keep your eyes on that spot of the floor as the camera moves backwards, you can tell there's no blood on the floor at all.

You're assuming it was blood he slipped in. The bloody hand print could simply be there to indicate violence ensued. But the movie never actually indicated Shaun slipped in blood. Since he slipped on something right in front of a cooler, where (by the blood stain) someone was attacked, it could easily have been water or clear soda. Also, if he had slipped in blood, he would have noticed, since he would have landed in it & gotten some on him.